Method for the production of substantially open-cell polystyrene sheet

Abstract

A method of producing a substantially open-cell, expanded polystyrene sheet, comprising the steps of:forming a mixture comprising from 30 to 95% of polystyrene incorporating an aliphatic hydrocarbon with 4-6 carbon atoms, from 0 to 65% of polystyrene, and from 0.2 to 10% of a nucleating agent,mixing the mixture obtained and melting it by heating inside an extruder,bringing the mixture to a temperature of 130-150° C. in the final portion of the extruder,extruding the mixture in a lower-pressure atmosphere to produce an expanded sheet, which has a low density and a high capacity to absorb aqueous liquids, particularly if a surfactant is added to the starting mixture, and which, when coupled with two films of plastics material gives rise to a laminate suitable for the production, by thermoforming, of trays of foods which may release aqueous liquids.

We claim:  
     
       1. A method of producing a substantially open-cell polystyrene sheet, comprising the steps of: 
       (a) forming a mixture comprised of (i) from 30 to 95% of polystyrene beads which contain an aliphatic hydrocarbon with 4-6 carbon atoms, (ii) from 0 to 65% of virgin polystyrene beads, and (iii) from 0.2 to 10% of a nucleating agent,  
       (b) forming a melt of the mixture obtained in step (a) by heating the mixture inside an extruder  
       (c) bringing the melted mixture to a temperature of 130-150° C. in the final portion of the extruder, and  
       (d) extruding the melted mixture in a lower-pressure atmosphere to produce an expanded sheet thereof.  
     
     
       2. A method according to  claim 1 , in which the mixture comprises from 5 to 10% of the nucleating agent. 
     
     
       3. A method according to  claim 2 , wherein the aliphatic hydrocarbon with 4-6 carbon atoms constitutes 1-7% by weight of the total weight of the mixture. 
     
     
       4. A method according to  claim 3 , wherein the aliphatic hydrocarbon with 4-6 carbon atoms is constituted by pentane. 
     
     
       5. A method according to  claim 2 , wherein the nucleating agent is selected from the group comprising sodium bicarbonate, citric acid, talc, sodium carbonate, Hydrocerol® and gypsum. 
     
     
       6. A method according to  claim 1 , wherein the mixture comprises a quantity variable from 0.2 to 10%, of the total weight of the mixture of at least one surfactant. 
     
     
       7. A method according to  claim 6 , wherein the at least one surfactant is a salt of a sulphonic acid of formula R—SO 3 H or of a sulphuric ester of formula R—OSO 3 H, in which R is selected from the group comprising alkyl and alkylaryl, with an alkali metal or an alkaline-earth metal. 
     
     
       8. A method according to  claim 1 , wherein the mixture comprises a quantity of up to 30%, of a shock-resistant polystyrene. 
     
     
       9. A method according to  claim 6 , wherein the mixture comprises a quantity of up to 30% of shock-resistant polystyrene. 
     
     
       10. A method according to  claim 6 , wherein the mixture comprises a quantity variable from 1 to 4% of the total weight of the mixture of at least one surfactant. 
     
     
       11. A method according to  claim 8 , wherein the mixture comprises a quantity of 10 to 20% of the shock-resistant polystyrene. 
     
     
       12. A method as defined in  claim 8 , wherein the shock-resistant polystyrene is a dispersion of polybutadiene in polystyrene. 
     
     
       13. A method as defined in  claim 11 , wherein the shock-resistant polystyrene is a dispersion of polybutadiene in polystyrene. 
     
     
       14. A method as defined in  claim 6 , wherein the mixture comprises a quantity of 10 to 20% of said shock-resistant polystyrene. 
     
     
       15. A method as defined in  claim 6 , wherein the shock-resistant polystyrene is a dispersion of polybutadiene in polystyrene. 
     
     
       16. A method as defined in  claim 14 , wherein the shock-resistant polystyrene is a dispersion of polybutadiene in polystyrene.